The crystal and defect structures of polar KBiNbO.

KBiNbO was prepared from RbBiNbO by a sequence of cation exchange reactions which first convert RbBiNbO to LiBiNbO, before KBiNbO is formed by a further K-for-Li cation exchange. A combination of neutron, synchrotron X-ray and electron diffraction data reveal that KBiNbO adopts a polar, layered, perovskite structure (space group 11) in which the BiNbO layers are stacked in a (0, ½, ) arrangement, with the K cations located in half of the available 10-coordinate interlayer cation sites. The inversion symmetry of the phase is broken by a large displacement of the Bi cations parallel to the -axis. HAADF-STEM images reveal that KBiNbO exhibits frequent stacking faults which convert the (0, ½, ) layer stacking to (½, 0, ) stacking and , essentially switching the - and -axes of the material. By fitting the complex diffraction peak shape of the SXRD data collected from KBiNbO it is estimated that each layer has approximately a 9% chance of being defective - a high level which is attributed to the lack of cooperative NbO tilting in the material, which limits the lattice strain associated with each fault.